Heat sinks of the prior art use a variety of fin shapes for natural convection of heat to a cooling fluid, typically ambient air. The air passages formed by the fins are usually either rectangular or cylindrical, although there are also heat sinks with pins instead of fins. Finite-element computation packages can accurately predict thermal performance for heat sinks in a far wider variety of conditions than could ever be covered by actual testing. Although mostly mounted in specified orientations, most heat sinks would be expected to function adequately when oriented otherwise.
There is one situation, however, where heat sinks have been found wanting, namely when attempting to cool electronic equipment within a hot, sometimes insulated compartment. Such is the case with light-emitting diode (LED) downlights. Incandescent light bulbs are less temperature-sensitive than the LEDs seeking to replace them in downlights. Currently, the maximum long-term operating junction temperature of commercially available LEDs is approximately 175° C. (Nichia of Japan). Such temperatures, however, significantly degrade efficacy. A junction temperature near ordinary room temperature typically achieves the highest efficiency, but anything at or less than 60° C. suffices. Thus an ambient temperature of 25° C. requires an LED downlight's thermal management system to achieve a 35° C. temperature delta. Some active LED cooling systems are available on the market that can achieve this. Nuventix of Texas markets products for this purpose under the name Synjet, but costs are very high (in some cases more than the price of an LED downlight). Also, these units have moving parts, which can potentially break down with time, and require significant operating power. For example their product for cooling an LED PAR 38 lamp requires 1.5 watts of power. Finally, the bulk, geometry, and configuration of this company's products severely limit their use.
Downlights in general are mounted inside a downward-facing ceiling-can, with a 6″ (15 cm) diameter being most common. Most commercial cans are open at the bottom but sealed laterally, and surrounded by ceiling insulation at that. Thus, hot air is trapped in them, a condition that prevents conventional heat sinks from being cooled by ambient air. A heat sink for LEDs that is adequate in ambient air will not work once the stagnant air in the can gets significantly above ambient, something that occurs when only a few Watts of waste heat are sent into an insulated can that is open only downwards. The majority of current commercial downlights typically contain 75-250 W incandescent lamps, corresponding to LED power levels of about 15-50 Watts electrical power consumption. About three quarters of the total electrical power, roughly between 10 and 40 Watts, is typically dissipated as heat within the can. The low-Watt limit on heat dissipation is thus a practical problem. Although cold-cathode fluorescent (CCF) lamps will also reduce power consumption and last longer, their relatively large size makes them optically inefficient within most cans. The hot air within a can will reduce the lifetime of CCF lamps as well.
LED downlights should out-compete compact fluorescent lamps, due to their compactness, ruggedness, and greater optical efficiency, except that the low-Watt limitation imposed by the can's insulation will greatly limit market penetration. Some companies have resorted to active means such as fans that bring ambient-temperature air to the heat sink of their LED downlights. What are needed are ways to move hot air out of the can without an apparatus of moving parts, with the attendant issues of cost, reliability, and noise.
Beyond LED downlights, LED light bulbs will need heat sinks that operate both horizontally and vertically, either within an envelope or externally. The development of LED light bulbs that are plug-compatible with existing incandescent lamps (such as the A19 lamps) has been hampered due to the limitations of existing thermal cooling technologies. Currently, system wattage is limited to between 2 and at most 5 Watts total system power. Given that the current system efficiency of high color rendition index (CRI) warm white LEDs is around 60 lumens per Watt, this limits the flux of these lamps to less than 300 lumens, only the output of a standard 25 W lamp. This figure is only achievable under ideal conditions, when the room temperature is 25° C. and the lamp is installed in a relatively open air light fixture. In worse (i.e., hotter) conditions the maximum practical output of today's LED light bulbs is closer to 200 lumens, severely limiting their market penetration.